Thank you all for replying, I'm glad that I'm not the only one with a baby that just doesn't seem to want to nap for long! He is now 5 months old, so maybe he will start to take longer naps as he gets more active!
This morning was a nightmare, he woke at 6.20, I fed him at 6.40, he was tired at 7.50 so I put him to bed and he went to sleep by 8.05, then he was awake again at 8.40! He didn't cry, so I left him for 30 mins while he chattered and played. I tried to intervene at that point by turning him back onto his back but he just didn't seem tired at all!
That meant he was then sleepy when I fed him at 10.40! I have just put him back to sleep again at 11.45.... I just know he will be a wake again at 12.15. Maybe I should try the 'wake to sleep' idea... did any of you try this approach?